1. Field of the Invention
The present invention relates generally to a printing apparatus for printing full color images through a superposing transfer of different color images effected by a plurality of electrostatic recording units each having an electrophotographic record printing function, and more particularly to a printing apparatus ensuring appropriate control of the quantities of development and consumption of color toners by the plurality of electrostatic recording units.
2. Description of the Related Art
Up until now, a color printing apparatus using electrophotographic recording comprises four electrostatic recording units corresponding to four colors, black (K), cyan (C), magenta (M) and yellow (Y) and arranged in tandem in the direction in which record papers are conveyed. The electrostatic recording units for four colors optically scan a photosensitive drum on the basis of image data to form latent images thereon, develop these latent images with color toners stored in respective developing vessels, and transfer them onto a record paper being conveyed at a certain speed in a superposed manner in the order of yellow (Y), magenta (M), cyan (C) and black (K), the resultant images being finally subjected to thermal fixing by a fixing device. In case of exhaustion of the color toners in the electrostatic recording units for yellow (Y), magenta (M), cyan (C) and black (K), there is a need for replacement of the entirety of the unit(s) or part of the unit(s). For this reason, the electrostatic recording units each have a structure allowing easy mounting and dismounting with the apparatus cover opened. Although in such a color printing apparatus comprising the Y, M, C and K electrostatic recording units arranged in tandem in the conveyance direction of the recording papers, toner units are loaded for operation which stores therein Y, M, C and K color toners by specified quantities, the quantities of toners consumed through the print operation may differ depending on colors. This is attributable to the fact that color toner components are consumed depending on the content of a color image printed. Furthermore, even though the consumption rates of the color toners used in the color image printing are the same, the toner developing quantities in the Y, M, C and K electrostatic recording units may be uneven, resulting in different toner consuming quantities from color to color. Consequently, when the residue of a most consumed color toner has reached a specified value or below, the other color toner cartridges containing sufficient residues required for development must also be replaced with new one, which results in a shorted toner life and hence in increased running costs.
Furthermore, in the case of the color printing apparatus comprising the Y, M, C and K electrostatic recording units arranged in tandem in the conveyance direction of the record papers, the transfer of the developing color toners is carried out by the Y, M, C and K electrostatic recording units while conveying the record papers adhered by suction onto the surface of the endless belt. Due to its large width exceeding the paper size, the conveyance belt may be gradually offset in the transverse direction orthogonal to the conveyance direction, during its travel around the rollers, owing to deflection of the belt or uneven tension. If this belt offset remains left as it is, the belt may diverge from the transfer positions on the photosensitive drum or may cause a breakage as a result of contact of the belt end faces with the apparatus interior. Thus, it is also conceivable to provide a structure for restricting the displacement of the belt by flanging the roller ends. However, once the belt starts to be offset, the offset could not be suppressed irrespective of the presence of the flanges due to the large width of the belt, resulting in a breakage of the belt.